Common practice to set the output voltage of a switching converter is to close the control loop using an error amplifier which compares the target reference voltage and output voltage; whenever a delta between the two inputs of the error amplifier occurs, the error amplifier activates the feedback control forcing the output voltage high or low accordingly until the error is cancelled;
Disadvantages of this practice are factors that are not taken into account and cannot be compensated such the effects of the load regulation which occurs at large load currents and is caused by the parasitic resistance in the control loop (e.g. parasitic resistance of the coil, switch resistance of the solid state switches or rectifier diodes, resistance of the PCB tracks, etc.).
The effect on the load regulation can be partially compensated with trimming of the static reference (but resistance is supposed to change with temperature and aging of the components) or very high gain in the control loop which has the loss of the control stability as main drawback;
The problem becomes relevant when the regulation error has to be minimized on account to a good accuracy in switching converter regulators when the non-idealities present in the control loop are considered.
An example of a practical application problem is in a backlight system when the error in the regulated voltage could be an issue to optimize efficiency.
FIG. 1a prior art shows a typical implementation of a backlight system which comprises a boost converter 1, a set of strings of LEDs 2 each of which has a programmable current source (IDAC) 3 which are controlled by a DAC controlled block 4.
Feedback voltage 5 is taken at the bottom of the string of LEDs and sent to the error amplifier 6.
The dotted boxes 7-10 show where the major sources of parasitic resistance can be identified in the loop (parasitic resistance in the inductor 7, diode 8, switching device 9, and PCB line 10 connecting the LED strings 2.
In this specific example in order to operate at maximum efficiency, the voltage IDAC_FB at the top of the programmable current source 3 has to be regulated as close as possible to the minimum voltage that still guarantees saturation of the output stage (saturation voltage) of the IDAC current source.
If the regulation is too low, the current source goes into triode and does not deliver the programmed current, if too high the saturation is guaranteed but at the expenses of a lower efficiency.
Similar to the parasitic resistance, also the saturation voltage changes with temperatures as well as with the process variations of the silicon and the supply voltage, therefore in this example an optimal setting of the reference voltage at a specific load and temperature condition will not suit for all conditions;
It is a challenge for a designer of switching converters to overcome the problems caused by non-idealities of the circuit.